In the area of water-jet surgery—this is an especially important example —in general the following requirements must be met:                The sterility of the fluid must be ensured, because it not only comes into contact with a patient's body but also to some extent remains in the body;        the required flow and/or the pressure needed for such flow must be substantially constant and reproducibly adjustable, to achieve a reproducible effect on the tissue;        the person operating the device must be free to select the duration of an individual activation with constant flow;        containers for the fluid should be available in differing sizes, so that for a particular application the amount required can be kept in reserve, with no need to exchange the container during an application on one hand and, on the other hand, without leaving a too-large container only partially emptied;        it must be possible to begin and end the activation of the instrument substantially without delay, i.e. the pressure must build up rapidly and no drops may appear post-activity;        it must be possible to exchange applicators with no problems.        
Surgical water-jet cutting devices and associated systems for pressure build-up are known per se. One technical solution to the problem of keeping the operating fluid completely sterile, namely by preventing it from coming into any contact with parts of the pump mechanism, is disclosed by the document DE 42 00 976 C2. With this known solution, however, the user has available only a specific quantity of the fluid medium, because it is contained in a previously prepared cartridge. Once this has been opened, any content that remains cannot be put to further use, regardless of how much or how little has been extracted during the actual operation. The residue must be discarded. The maximal size of the reservoir in this known arrangement is crucially determined by the dimensions of the device. When a small device is desired, it can easily happen that the reservoir must be exchanged during the operation, which constitutes a disturbing interruption of the procedure. Furthermore, the reservoir must be pressurized as a whole, so that there is a dependence on the sizes that are available for the reservoir.
Sterile filters, which allow nonsterile transport in that they filter out any germs that could have entered the cutting medium from a nonsterile transport device, present various disadvantages. One is that the effort of maintenance is considerably increased, because the filters must be exchanged at specified time intervals. Furthermore, filters tend to inhibit the flow behavior. In the case of high pressures, inserted sterile filters are suitable only with some restrictions, because when the instrument is turned on and off they are subjected to major changes in flow and thus also interfere with the required rapid pressure build-up.